1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to data transmission, and more particularly, to data transmission which applies an appropriate coding rate according to significance of bits or bit groups included in uncompressed data and retransmit all or part of the data when a transmission error occurs in the data while the data is being transmitted over a wireless network.
2. Description of the Related Art
As networks become wireless and the demand for large multimedia data transmission increases, there is a need for studies on an effective transmission method in a wireless network environment. In particular, the need for various home devices to wirelessly transmit high-quality videos, such as digital video disk (DVD) images or high definition television (HDTV) images, is growing.
An IEEE 802.15.3c task group is developing a technological standard for transmitting large-volume data over a wireless home network. The technological standard, which is called “millimeter wave” (mmWave), uses an electric wave having a physical wavelength of a millimeter (i.e., an electric wave having a frequency band of 30-300 GHz) to transmit large-volume data. This frequency band, which is an unlicensed band, has conventionally been used by communication service providers or used for limited purposes, such as observing electric waves or preventing vehicle collision.
FIG. 1 is a diagram comparing frequency bands of IEEE 802.11 series standards and mmWave. Referring to FIG. 1, the IEEE 802.11b or IEEE 802.11g standard uses a carrier frequency of 2.4 GHz and has a channel bandwidth of approximately 20 MHz. In addition, an IEEE 802.11a or IEEE 802.11n standard uses a carrier frequency of 5 GHz and has a channel bandwidth of approximately 20 MHz. On the other hand, mmWave uses a carrier frequency of 60 GHz and has a channel bandwidth of approximately 0.5-2.5 GHz. Therefore, it can be understood that mmWave has a far greater carrier frequency and channel bandwidth than the related art IEEE 802.11 series standards. When a high-frequency signal (a millimeter wave) having a millimeter wavelength is used, a very high transmission rate of several Gbps can be achieved. Since the size of an antenna can also be reduced to less than 1.5 mm, a single chip including the antenna can be implemented. Furthermore, interference between devices can be reduced due to a very high attenuation ratio of the high-frequency signal in the air.
In recent years, a technique for transmitting uncompressed audio or video data (hereinafter, referred to as uncompressed data) between wireless apparatuses using the millimeter wave having a large bandwidth has been studied. Compressed data is compressed with a partial loss through processes such as motion compensation, discrete cosine transform (DCT) conversion, quantization, and variable length coding, such that portions of the data insensitive to the sense of sight or the sense of hearing of human beings are eliminated. In contrast, uncompressed data includes digital values (for example, R, G, and B components) representing pixel components.
Hence, bits included in the uncompressed data have different degrees of significance while there is no difference in the significance of bits included in the compressed data. For example, referring to FIG. 2, a pixel component of an eight-bit image is represented by eight bits. Of the eight bits, a bit representing the highest order (the highest-level bit) is the most significant bit (MSB), and a bit representing the lowest order (the lowest-level bit) is the least significant bit (LSB). In other words, each of eight bits that form one-byte data has a different significance in restoring an image or audio signal.
When an error occurs in a bit having high significance during transmission, it is possible to detect the error easier than when the error occurs in a bit having low significance. Therefore, it is necessary to protect bit data having high significance such that no error occurs in the bit data during wireless transmission, as compared to bit data having low significance. A method of correcting errors of all bits to be transmitted at the same code rate, which is a related art transmission method, has been used in the IEEE 802.11 standard.
FIG. 3 is a diagram illustrating the structure of a physical (PHY) protocol data unit (PPDU) 30 of the IEEE 802.11a standard. Referring to FIG. 3, the PPDU 30 is composed of a preamble, a signal field, and a data field. The preamble, which is a signal for PHY layer synchronization and channel estimation, is composed of a plurality of short training signals and a long training signal. The signal field includes a RATE field indicating a transmission rate and a LENGTH field indicating the length of the PPDU 30. Generally, the signal field is encoded by a symbol. The data field includes a physical layer service data unit (PSDU), a tail bit, and a pad bit. Data to be transmitted is included in the PSDU.
Data recorded in the PSDU is composed of codes that are encoded using a convolution encoder. Bits that form data, such as compressed data, are not different in terms of significance. In addition, since the bits are encoded using the same error correction encoding method, an equal error correction capability is applied to each bit.
This related art data transmission method can be effective for general data transmission. However, if each portion of data to be transmitted has a different significance, it is necessary to perform more superior error correction encoding on portions of greater significance in order to reduce the probability of error occurrence.
In order to prevent error occurrence, a transmitting end performs error correction encoding on data. Even if an error occurs while the error-correction encoded data is transmitted, the error-correction encoded data can be restored as long as the error is within a correctable range. There are a variety of error correction encoding algorithms, and each error correction encoding algorithm has a different error correction capability. Even the same error correction encoding algorithm may show different performances depending on a coding rate used.
In general, as the coding rate increases, data transmission efficiency is enhanced, but error correction capability is reduced. Conversely, as the coding rate decreases, data transmission efficiency is reduced, but error correction capability is enhanced. As described above, since uncompressed data includes bits having different degrees of significance unlike compressed data, upper bits, which are more significant than lower bits, need to be better protected against errors during data transmission.
Related art methods of guaranteeing stable wireless data transmission include a method of restoring data using error correction encoding and a method of re-transmitting data having an error from a transmitting end to a receiving end.